Swampie....
Savage added the .243 and .358 Winchester calibers to their Models 99R, RS, EG in 1956 and the "F" model in 1959.
To give you an age, I'd need the rifle's serial number and it's Lever Boss Code (if it has one).
The Lever Boss Code ("LBC") is located in a small, lightly-stamped oval on the front, lower side of the "lever boss" which is that extension on the lower, front side of the receiver into which the lever is fitted and rotates.
You will find a VERY lightly stamped oval, a number (the inspector's number) and a letter (A through X). There may be only a partial oval and/or LBC inside the oval. Use a magnifying glass if you have one.
The LBC is a more accurate method of indicating the year of manufacture... even more accurate than the rifle's serial number.
As concerns it's value... if the rifle is in reasonably good condition... it's EASLY worth $400 regardless of what model it is!!! Buy it QUICK 'cause they ain't makin' any more of 'em.
Hairtrigger......
Yes, in the last year Savage manufactured rifles at their plant in Chicopee Falls, Mass. (before they moved to Westfield, Mass. in 1960), they made some Model 99s in .243 with serial numbers in the 900,000's... I.E. , "pre-mils".
Strength & Honor...
Ron T.